The term “response time” as used herein refers to the mixing time or mixing energy necessary for obtaining a “target” slump (desired level or state of fluidity) of wet concrete in a rotatable mixer drum. The response time is frequently described in terms of the number of mixer drum revolutions required for the wet concrete to obtain a target slump after a fluidizing admixture is dispensed into the mixer drum.
A ready-mix concrete delivery truck may need to rotate the mixer drum 15-20 revolutions or more in order to mix water or naphthalene sulfonate type dispersants uniformly throughout a concrete load before the target slump is attained. Polycarboxylate type superplasticizers may require 50-70 or more revolutions of the concrete mixer drum before attaining complete and uniform dispersal within the concrete mix load. This behavior requires considerable planning and foresight in delivering concrete having a desired slump at the pour event.
While the concrete delivery truck is travelling on the road to the pour site, the maximum revolutions per minute (rpm) of the mixer drum is about 4-5 rpm. When the truck is stationary (parked), the loaded concrete mixer drum can rotate at a maximum rate of around 20-40 rpm. If one seeks to reach a target slump (a specified slump level) during travel and before the mix load reaches the pour site, one may need to introduce a polycarboxylate superplasticizer into the concrete mixer drum about 20 minutes or more before arriving at the construction/pour site in order to have sufficient processing time for reaching the target slump. Alternatively, when the mixer truck arrives at the construction/pour site, the admixture can be introduced into the concrete and the mixer drum rate can stepped up to 20-40 rpm while the truck is parked so that the fluidizing admixture can be mixed dispersed uniformly throughout the load, theoretically, within minutes.
However, concrete delivery trucks typically require 50-70 rotations of the mixer drum, or more, before an accurate slump measurement is obtained, in accordance with current practice. Before on-board slump monitoring systems were available, concrete slump was measured using the standard slump cone procedure on mix sample taken from the mixer. The use of slump cone measurement on the job site is still the usual practice, as the industry grapples with the number of variables—temperature, moisture and humidity, environment, quality of day to day materials—which can defeat accurate monitoring and chemical dosing even when automated slump monitoring systems are used.
In U.S. Pat. No. 8,058,377 B1 which issued on Nov. 14, 2011, and which is owned by the common assignee hereof, Goc-Maciewjeska et al. disclosed phosphate-containing polycarboxylate polymer dispersants which contained mono-ester, di-ester, and tri-ester groups in specific molar ratios for achieving “quick mix-in dispersibility” in comparison with polycarboxylate polymer dispersants that did not contain phosphate groups. Such phosphate-containing polymers conferred improved “initial” slump as well as slump “retention” to the concrete mix. Goc-Maciewjeska et al. believed that the mix dispersibility of these polymers would be “particularly useful in ready-mix delivery trucks or in plant batching operations whereby “time can be saved by the ability of the polymer dispersant to become uniformly distributed throughout a concrete mix in less time compared to polycarboxylate polymers that do not contain phosphate groups” (See U.S. Pat. No. 8,058,377 at col. 2, II. 23-29).
Goc-Maciewjeska et al. explained that this “relatively quicker mix dispersibility will be highly useful in automated mixer systems wherein the slump of concrete is monitored within a rotating drum mixer and adjusted by dosing a rheology-modifying agent (e.g., chemical admixture such as a water reducer or superplasticizer) into the mix.” (See U.S. Pat. No. 8,058,377 at column 2, lines 30-35). They further explained that after dosing the admixture and mixing the concrete, the energy required to turn the mixer is monitored until the energy curve flattens over time, thereby indicating that the dose has been uniformly dispersed within the mix. They found that certain phosphate-containing polymers dispersed more quickly in the concrete mix compared to polycarboxylate polymers that did not contain such phosphate groups, thereby shortening the time needed in automated slump monitoring operations. (See U.S. Pat. No. 8,058,377 at column 2, lines 35-42).
There is a need for fast response (“mix-in” dispersibility) time for polycarboxylate fluidizing polymers used in automated systems that measure the energy required to turn the rotatable mixer drum until the energy curve flattens over time (thereby indicating that the admixture dose is uniformly dispersed within the mix), because dosing and monitoring are performed in an iterative fashion that extends the theoretical mix-in time for dosing fluidizing admixture into concrete. A first admixture dose is dispensed into the concrete, and slump is monitored by the automated system over time (i.e., over a number of mixer drum revolutions) and then the slump data readings are compared to pre-recorded data stored in processor-accessible memory; and then one or more subsequent doses of the fluidizing admixture are dispensed into the concrete where discrepancy is detected by the system between actual and target slump levels; and then the concrete load is again monitored over further drum revolutions until target slump is attained. If a sufficient discrepancy continues to exist, the process can be repeated one or more times. Such automated slump monitoring systems are disclosed, for example, in U.S. Pat. Nos. 8,311,678 and 8,491,717 (Verifi, LLC), incorporated herein by reference.
“Response time” has not merited serious consideration as a primary or even desirable attribute of comb-shaped polycarboxylate ether (“PCE”) cement dispersants until the present invention.
It is thus an objective of the present invention to provide a method and system for achieving fast response time using one or more polycarboxylate ether polymer cement dispersants having a cumulative absorptivity coefficient of 40%-75%, thereby enabling slump monitoring systems to operate faster and more efficiently. Because automated slump monitoring systems must sample the concrete by rotating the drum a number of times to sample the energy required to rotate the drum and translate this into slump reading, and then rotate the drum a number of times to mix in a fluidizing admixture, the numerous drum revolutions can entrain too much air into the concrete mix and can also overheat the cement due to the grinding action of the aggregates. Thus, for the present inventors, achieving a fast response time is particularly desirable when the slump monitoring system must operate in iterative fashion (monitoring, adjusting slump, monitoring, adjusting slump, etc.) that might otherwise tend to degrade the delivered concrete.